Generally, transfer presses are equipped with a transfer system for transferring a workpiece in and out of a work station in timed relation with performance of a sequence of pressing operations. Such a transfer system includes a pair of juxtaposed feed bars extending in a workpiece feed direction and cross bars each spanned between these feed bars. A known transfer system conveys workpieces held by vacuum caps by vacuum adsorption, these vacuum caps being attached to the cross bars. Another known transfer system conveys workpieces gripped at both sides by fingers attached to the feed bars. In this case, the pair of feed bars perform two-dimensional or three-dimensional movement to transfer an individual workpiece from one station where a set of dies are disposed to the next adjacent station where another set of dies are disposed.
The most typical method for driving the feed bars is a mechanical driving method in which the feed bars are driven, being linked to the press system with a cam and link mechanism. This method is characterized in that the movement of the feed bars in a feed direction is determined by the feed cams whereas the movement of the feed bars in a lift direction is determined by the lift cams and the aforesaid two-dimensional movement is achieved by combining the movements in the feed and lift directions.
The mechanical driving method, however, reveals the following drawbacks.
(1) The complicated driving system makes maintenance difficult. PA1 (2) The feed bars cannot be supported at more than three positions because of limited space, which requires increased rigidity in the feed bars in order to restrict mechanical vibration. This causes an increase in the weight of the feed bars and, in consequence, an increase in the size of the driving system for the feed bars. PA1 (3) Since the driving system for the feed bars is long, the rigidity of the driving system needs to be increased in order to ensure accuracy in positioning each feed bar. PA1 (4) The feed bars need to be retracted in an upward direction to let the moving bolster out of the press system during replacement of dies, which gives rise to a need for forced lift cylinders. PA1 (5) The installation level of the feed bars is not adjustable according to dies. PA1 (6) The movement of the feed bars is fixedly determined by the cams so that the movement cannot be optimized according to dies to be used. PA1 (a) a cam angle detector attached to a cam shaft for the feed cams; PA1 (b) motion table setting means for setting, according to input data entered from outside, a motion table indicative of a locus of movement of the feed bars; and PA1 (c) controller means for controlling respective servo motors for lift bars according to information which is related to the cam angle of the feed cams detected by the cam angle detector and looked up in the motion table set by the motion table setting means.
As an attempt to solve these problems, Japanese Patent Publication (KOKAI) No. 6-262280 (1994) discloses a method for driving the feed bars in a lift direction with motors (servo motors) different from the motor for the press system. In the transfer feeder according to this publication, a pair of lift beams are driven vertically with a lift mechanism having servo motors as a driving source, and cross bar carries attached to the lift beams are driven in a feed direction with a feed mechanism having feed cams as a driving source, the feed cams being rotated by power taken out of the main body of the press system.
Transfer presses having such a transfer feeder are designed to detect the cam angle of the feed cams by a cam angle detector and control each servo motor based on the detected cam angle, that is, the press angle, thereby to drive the transfer system in synchronization with the motion of the press system.
The prior art transfer press of the above type has a difficulty in driving the transfer system in a lift direction when solely driving the transfer system with the feed cams disconnected from the press system, because the cam angle detector is installed on the side of a main motor for driving the press slide.
In addition, the prior art transfer press presents another disadvantage in its structure wherein lift beams are suspended from above using rack bars incorporated in the lift mechanism, which involves large-sized servo motors due to the influence of gravity and therefore leads to large installation space as well as an increase in production cost.
The present invention is directed to overcoming the foregoing problems and it is therefore one of the objects of the invention to provide a controller for a work transfer system which is capable of precisely positioning the feed bars even when the transfer system is solely driven and simplified in its structure to contribute to savings in installation space and cost.